1. Field of the Invention
The present invention relates to vehicle braking systems and is concerned in particular with an apparatus and method for assessing and controlling the pressure level within an accumulator from a pumped supply in such braking systems.
The present apparatus and method can be applied, inter alia, not only to pneumatic braking systems where a compressor charges air tanks of the braking system but also, as in the examples described hereinafter, to systems wherein a fluid pump is arranged to charge a high-pressure fluid accumulator. In both of these cases, the stored volume of working fluid is used by a brake control system to apply the brakes of the vehicle, under the control of an electronic control unit.
The brake control system is required to ensure that a volume of working fluid is available for operating the brakes and which remains within a prescribed pressure window, ie. between upper and lower thresholds of pressure. When the system is operated, the stored volume within the accumulator is depleted and needs to be replenished by pumping additional fluid from a reservoir (or from atmosphere in the pneumatic case) using a suitable pump, driven by an electric motor.
2. Discussion of the Background
Conventionally, the pressure within the accumulator is monitored and measured by means of a pressure transducer or pressure switch. When the pressure within the accumulator drops below a prescribed minimum level, the electronic control unit switches on the electric motor driving the fluid pump and leaves this motor running until the accumulator pressure has risen to a prescribed upper or maximum pressure level. However, the use of such pressure transducers or pressure switches adds significantly to the cost of such braking systems, particularly where the window or pressure range over which the pressure is being controlled is small, since the cost of suitable transducers or switches with appropriate accuracy is high.
It is also known, for example from GB-A-2225874, that a measure of the prevailing accumulator pressure can be gained by monitoring the power/current absorption of the electric motor driving the pump whilst it is running. As the pumped pressure increases, so the torque that must be applied by the motor to the pump shaft also increases. This can be established by measuring the motor current so that an inferred pressure level can be obtained. However, with such systems, a pressure transducer or other switching device is still required in order to determine the switch-on pressure level for the motor since, according to such systems, the pressure level can only be monitored during actual running of the motor.